Atopic dermatitis (AD) is a highly pruritic chronic inflammatory skin disease that is associated with significant morbidity and occupational disability affecting 17% of children and nearly 2% of adults in the U.S. Recent studies document significant morbidity from this skin disease and indicate that poorly controlled AD ,may be a harbinger of asthma in later childhood, i.e. the atopic march. Numerous studies have highlighted the importance of Staphylococcus aureus infection and colonization in exacerbating AD. S. aureus contributes to AD via the production of toxins which activate keratinocytes, professional antigen-presenting cells and T cells. The overall goal of this revised competing renewal R01 grant application (5 R01 AR41256-14) will be to determine why patients with AD are prone to colonization or infection with S. aureus. As well, we wish to continue studies investigating novel mechanisms by which S. aureus contributes to S. aureus skin inflammation. New preliminary data show that keratinocytes have an innate capacity to vigorously ingest and kill S. aureus. We hypothesize that increased IL-4/IL-13 in the skin of extrinsic AD (EAD) patients inhibits the capacity of keratinocytes to phagocytosize and kill S. aureus in EAD. This lack of killing is due to deficient expression of antimicrobial peptides (AMPs). However, an alternative non-IL- 4/IL-13 mediated mechanism must exist for patients with intrinsic AD (IAD). The inability of AD skin to eradicate superantigen-producing S. aureus appears to result in the subversion of T regulatory cell activity required for control of skin inflammation. The specific aims of this will be: first, to determine the mechanisms by which keratinocytes kill S. aureus, focusing on the role of phagocytosis and specific microbicidal mechanisms; second, to assess how the microbicidal activity of human keratinocytes is regulated by TH1 and TH2 cytokines; third.' to determine whether skin of IAD patients are deficient in AMP expression and the potential effect of IL-10 on AMP production; finally, to investigate the mechanism by which staphylococcal superantigens subvert T regulatory cell activity and explore their role in modulating AD skin inflammation. These studies should provide new insights into the mechanisms for increased S. aureus infection in AD and the strategies by which S. aureus amplifies AD skin inflammation. These studies are likely to identify novel therapeutic approaches in the control of S. aureus infection and modulation of T regulatory cell activity.